Data converters are used in many applications such as telephone switching equipment, voice-band data communications, speech coding systems, and audio and video signal processing equipment. Data converters have incorporated gain scaling of an incoming analog signal to reduce the output noise of an analog-to-digital converter. In a data converter, signals are converted from one form to another. One of the forms is analog and the other form is digital.
A data converter includes either an analog-to-digital (A/D) converter, a digital-to-analog (D/A) converter, or both. An A/D converter receives an analog signal and provides at its output a digital signal representative of the analog signal. A typical digital signal representation is linear pulse code modulation (PCM). A D/A converter receives a digital signal and provides at its output an analog signal. PCM encoding in data converters results in a one-to-one mapping of analog signal amplitudes to corresponding PCM word values. The analog circuits of data converters have a predetermined amplitude range over which they operate linearly. The predetermined upper and lower bound of this range establishes the analog levels which correspond, respectively, to the maximum and minimum PCM code values for the data converter.
Sigma-delta data converters employ an intermediate step in the conversion process known as sigma-delta modulation, which creates an encoded data stream having a pulse code density proportional to the amplitude of the analog input signal. A sigma-delta A/D converter first converts an analog input into a pulse code density stream, then converts the pulse code density stream into a corresponding PCM word. The pulse code density is proportional to a predetermined maximum and minimum analog input. As the pulse code density approaches a unity value, the signal-to-noise ratio degrades significantly. When this pulse code density stream is converted into PCM words, the PCM words will have a significantly degraded signal-to-noise ratio in the range of its full-scale limit.
Similarly, a sigma-delta D/A converter first converts a series of PCM input words into a pulse code density data stream, then converts the pulse code density stream into a corresponding analog output signal. As the PCM input words approach their maximum levels, the pulse code density of the data stream approaches unity value, resulting in a heavily degraded signal-to-noise ratio. When this pulse code density is converted into an analog output signal, the analog output signal will have a heavily degraded signal-to-noise ratio before it reaches its predetermined full-scale limit.
For full-scale signal magnitudes, a sigma-delta modulator employed as a converter will produce a distorted output necessitating that gain scaling be employed. Gain scaling of digital signals involves multiplication which can produce truncation or rounding bits of lower significance. Another source of distortion is introduced into the signal by the truncation or rounding to reduce word width and results in harmonic distortion of the input signal in the energy spectrum.
A need exists for a technique useful with gain scaling that reduces the distortion introduced by truncation or rounding in gain scaling.